Solid state switching device



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W. R. EUBANK ETAL SOLID STATE SWITCHING DEVICE Filed Sept. 2'?, 1965 VOL75 April 4,

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United States Patent O 3,312,924 SOLID STATE SWITCHING DEVICE William R.Eubank, VTroy Township, St. Croix County,

Wis., and Bavid L. Cross, St. Paul, Minn., jassgnors to Minnesota Miningand Manufacturing Company, St.

Paul, Minn., a corporation of Delaware Filed Sept. 27, 1965, Ser. No.490,519 6 Claims. (Cl. 338-20) This application is acontinuation-in-part of our earlier led application S.N. 376,483 ledJune 19, 1964 now abandoned.

This invention relates to new and very useful solid state semi-conductorpolarity sensitive switching devices and to electrical circuits and`methods `for using. such devices. The invention also relates to oneclass of glass compositions containing arsenic, phosphorus, sulfur andiodine useful in the manufacture of such devices. It has now beendiscovered that certain glass compositions, when suitably prepared intosemi-conductor devices, exhibit a distinctive and characteristicpolarity sensitive nonsymmetrical switching in response to an appliedelectric field. The invention is described in the followingspecification taken together with the drawings wherein:

FIGURE l is one embodiment of a semi-conductor switch construction ofthis invention;

FIGURE 2 is an enlarged vertical sectional view of the semi-conductorswitch device of FIGURE 1; e

FIGURE 3 is a plot of the Voltage-current characteristic associated witha device of this invention;

FIGURE 4 is one embodiment of a circuit diagram of an electricalcircuit'suitable for inducing conduction and for making electricalmeasurements upon a device of this invention; and

FIGURE 5 shows one embodiment of a schematic circuit diagram using adevice of this invention.

Devices of this invention use glasses which contain the elementsarsenic, sulfur and iodine, and, optionally, the elements antimony andphosphorus. Thus, the glasses useful in the invention can be consideredto comprise one ternary system, three quaternary systems and quinarysystem, as tabularized in the following Table I:

Patented Apr. 4, 1967 In general, the starting materials employed toproduce theV glasses used in devices of this invention either are thecomponent elements themselves or are compounds of two or more suchelements. When using uncombined elements, i-t is generally preferable toemploy each in a highly purified and iinely divided form. However,largely because of the high volatility of elemental iodine, it isconvenient and preferred to employ compounds of iodine in place ofiodine itself, for example AsI3.

Finely divided flowers of sulfur are found to be a convenient form ofthat element to use for making cornpositions. Granular or powderedanalytical grade arsenic, antimony or phosphorus are preferablyemployed. The sulfur and arsenic can be preferably prereacted in theratio to form As2S3 before addition of the ternary component andsubsequent formation of a glass, as described below.

In general, one can conveniently use two methods to prepare glasscompositions of thisinvention. One method involves melting startingmaterials in a closed tube, and the other involves melting startingmaterials or remelting a glass formed in the closed tube in an open tubeyin order to vapor deposit thin glass layers on an aluminum substrate. jl

The closed tube method for preparing glasses of this invention involvesmelting the starting materials within a suitable heat resistant sealedtube, as indicated..

The tube after sealing is then preferably suitably mounted for axialrotational movements in a hot zone maintained at a temperature of about800 C. or higher. Each sealed tube is approximately thus maintained insuch hot zone for about 1/2 to 1 hour -or until a homogeneous liquidmelt is obtained. Thereafter, the tube and melt therein are removed fromthe hot zone and allowed to cool slowly. If any crystallization isvisually observed, the tube and contents are remelted and then rapidlycooled.

When melting in an open tube, one can employ heat resistant glass tubesand deposit in each tube a measured premixed quantity of individuallyweighed desired start- TABLE I.-GLASSES SUITABLE FOR VAPOR DEPOSITION OFTHIN ORI- ENTED LAYERS 1 Compositional Range, Atomic Percent GlassSystem Phosphorus Arsenic Antimony Sulfur Iodine (P) (AS) (Sb) (S) (I)lTotal atomic percent of respective elements in any given always 100. K

2 In U.S. Pat. No. 3,024,119.

In copend-ng applicatlon S.N. 490,515.

The third quaternary system (glass system IV of Table I),arsenic-phosphorus-sulfur-iodine, and the one quinary system (glasssystem V of Table I) arsenic-phosphoruscomposition is ing materials.Each such tube is then immersed into a hot zone maintained at atemperature usually above about 500" C. and preferably a little aboveabout 550 C. Though times for the starting materials to melt and becomehomogeneous vary, they commonly range from about 1/2 to l hour, thoughlonger or shorter times may be Vexperienced depending on individualcircumstances. Stirring helps promote homogeneity. After a homogeneousmelt is obtained, the tube is removed from the hot zone and allowedtocool in air at room temperature, This cooling rate is generally slowerthan about 10 centigrade per second (10 C./sec.).

If one visually observes any crystallization in the melt as it thusslowly cools Within the tube, such tube can be reinselted into the hotzone and the mixture remelted. Then when the hot tube and contents areremoved from the hot zone,`they are'rapidly quenched, as my immersioninto water at room temperature or the like, so as to rapidly cool suchtube and contents at a rate greater than about 100 centigrade per second(100 C./sec.).

The sealed evacuated tube method for studying the glass-formingcharacteristics of the system studied is l preferred since possiblevolatilization losses, resulting in slight compositional changes duringmelting are thereby eliminated. Also higher temperatures, by about 100C., may be employed allowing solution of certain more diflicultlysoluble components to take place more readily. For practical purposessuch as application of thin layers of the glas-s by` vapor deposition onan aluminum substrate, however, it was necessary to employ the open tubemethod of melting. In many cases glasses made and characterized by theclosed tube method were remelted and obtained as thin layers by the opentube method.

Independently of the method of melting and associated cooling, aquenched melt is next examined to determine its state and to see ifglass formation has occurred.

For purposes of this invention in vdetermining whether or not a cooledsolid product is a glass, the following considerations are used:

(l) Presence of conchoidal fracture upon breaking of a sample.

(2) Substantially no birefringence when `a sample is eX- amined under apetrographic microscopeV (with a glass which is not too opaque for suchan examination).

(3) Substantially no distinct lines when a sample is examined by theX-ray powder diffraction technique. (4) Gradual softening and finalremelting of a sample as its temperature is increased (in contrast tothe sharp melting points characteristically observed in the case ofcrystallized materials).

(5) Pulling 1a long (eg. 2.5 feet (about 3% meter) or longer) liber froma sample of molten material smaller than about 1/2 gram before suchsample solidifies.

TABLE IIL-THIN ORIENTED VAPOR DEPOSITED By way of summary, glasscompositions useful in making devices of this invention comprisearsenic, sulfur and iodine in amounts as defined by Table I. Optionally,such composition can have the arsenic present therein partially replacedbyl at least one of the elements selected from the group consisting ofphosphorus up to .about 12 atomic percent and antimony up to about 35atomic percent (see Table I and Table II). A given composition should besuch that when it is heated to a temperature range of from about 500 to600 C., it has a vapor pressure in excess of l0 torr. In general, allglass compositions useful in this invention are characterized by havingan ability to Wet and adhere to an aluminum surface, said surface beingcharacterized by (l) having a roughness not more than about 25microinches (about 0.625 micron) r.rn.s. (root mean square),

(2) being substantially free of aluminum oxide,

(3) being at a temperature not more than about 500 C.

The novel glass compositions of this invention (glass systems IV and Vof Table I) are not only characterized by having an ability to wet andadhere to an aluminum surface as aforedescribed, but also arecharacterized by having a vapor pressure at temperatures in the rangeyof from about 500 to 600 C., in excess of about 10 torr. andv generallyin excess of the vapor pressures associated with other glasscompositions known to be useful in4 devices of this invention. Such highvapor pressures appear to be advantageous in depositing thin vaporcoatings on aluminum substrates and for such reason these compositionscan be considered a preferred type of glass composition lfor use inmaking devices of this invention. These novel glasses have a compositioncomprising from about 1 to 12 atomic percent phosphorus, from about 5 to40 atomic percent arsenic, from about 40 to 60 atomic percent sulfur,from about 2 to 22 atomic percent iodine, and, optionally, from about 0to 35 atomic percent antimony, the total atomic percent of allrespective elements in any given composition always being 100.

Devices of this invention employ glass compositions as above describedvapor deposited upon an aluminum metal substrate. Such substrates can bein any convenient physical shape. We nd aluminum sheeting to be thepreferred substrate material, apparently because of the preferredorientation of its surface crystals.

The surface of such a substrate upon which a glass GLASS LAYERS ONALUMINUM VSUBSTRATE 1 Starting Composition Relative Relative Vapor ofGlass (Atomic Percent) Fluidity of Pressure above 1 Appearance oi VaporDeposited Thin Film Y Example No. Melt at elt at on Aluminum SubstrateSwitching Characteristic 500 C. 500 C. P As Sb S I 15 High High Shiny,vitreous, continuous Good polar. 22 rln dn dn D0,

9 Intermediate.. Intermediate.. Vitreous, somewhat uneven `Polar buterratic. 3 Very low Low Vitreous but uneven D0.

15 High High Same as EX. 1 Good polar'. 15 rln do -.do Do. d do Do.

-.do symmetrical.

9 Vitreous, continuous but not very smooth. Switching only.

0 Fair polar. System IV 11 12 25 48 Vitreous but uneven Polar buterratic.

4 33 48 Vitreous, even Good polar. 1 36 45 do Do. 8 40 60 Do. 1 36 60...-dn Do. 15 20 40 Poor coating, dull and uneven Could not be switched.

4 15 18 48 .do Vitreous, even Good polar. 4' 35 5 54 Intermediate-.Vitreous, uneven. Polar but erratic. 2 5 30 41 22 igh 1gb Vitreous, evenGood polar. 10 10 15 48 15 rlo -....fln n i Do.

1 In these examples, the Al substrate consists of strips 0.4 mm.) thickhaving one surface finished to about 3-5 mi approximately 6 inches(about 15.25 om.) long, inch (about 1.27 cm.) wide, 16 mils (aboutcroinchcs (about (M-0.125 micron (l0) r.m.s. and freshly etched with 20%NHrOH solution.

composition is deposited needs to have certain characteristics. For onething, such surface should'be substantially free of aluminum oxides,such as those for-med in situ by exposure of an aluminum surface to air(oxygen).

As used in this invention the term substantially oxide free orequivalent language in reference to the surface of an aluminum substrateused in a switching device of this invention denotes the fact that suchsurface has no aluminum oxide thereon other than that in situ aluminumoxide which is produced by exposure of an aluminum surface to air atroom temperature (20 C.) for a period of time of about 12 hours afterthat aluminum surface has been first immersed in an aqueous solution of20% ammonium hydroxide for a period of time suflicient to remove any insitu aluminum oxide on such surface and thereafter removed from suchsolution, washed with water and dried. t

Before being vapor coated with a glass, an aluminum substrate surfaceshould be cleaned to remove aluminum oxide therefrom.

While such oxide can be removed -by various conventional means, it ispreferred for purposes of this invention to etch the surface of thealuminum by chemical treatment to remove said oxide layer. One preferredetching procedure which has been found entirely satisfactory forpurposes of this invention is as follows:

(1) Rinse the surface of the aluminum substrate in a solvent oftrichloroethylene or other suitable degreasing liquid.

(2) Dry the rinsed surface in air.

(3) Place the aluminum substrate surface into a bath of ammoniumhydroxide and distilled water to 20% NH4OH) and leave for about 5minutes. The surface, alternatively, can be kept in such bath until gasevolution is visually detected coming from the surface of the aluminum.

(4) Rinse the so-etched aluminum surface in three different baths ofdistilled water. The strips are suitably soaked in each bath for 5 to 10minutes at room temperature.

(5) Dry the washed surface in air. The strips can now be used to formdevices of the invention.

Also, for purposes of making devices of this invention, the surface ofthe aluminum substrate should have a roughness not greater than about 25microinches (about 0.625 micron (p.)) r.m.s. Preferably, such surfaceshould possess reliector grade properties to which reference is had tothe conventional trade designation of aluminum sheet metal having asurface roughness of not more than about 5 microinches (about 0.125micron (p0) r.m.s.

Thin layers of glass compositions as above described can be vapordeposited on an aluminum substrate as above described by anyconventional method.

However, one method which has been found t-o be especially satisfactoryand convenient involves beginning with a glass prepared, for example, ina closed t-ube as above described. Then one remelts same in an open tubein which only the bottom part of the tube is heated and the volume ofthe glass in the hot zone is small compared to the total volume of thetube so that the glass when melted at a temperature above 500 C. has asubstantial vapor pressure above thev liquid thereby effectivelyreplacing a large percentage of air in the tube. Such a highconcentration of vapors readily condenses on a cool (i.e. initially roomtemperature) aluminum substrate surface having a smoothness and oxidefreeness as above described. t

Such aluminum s-ubstrate typically in the form of a strip of sheetAmetal is held in the vapors above the melt of the glass for a period oftime suicient to deposit an 6 integral continuous film. Usually thisrequires two to three minutes. During this period the temperature of the`aluminum substrate surface may rise several hundred degrees centigradebut generally Vremains substantially below the temperature of that ofthe molten glass.

After a desired quantity of the vapors have been deposited on thesubstrate as a smooth glass layer, the socoated substrate is withdrawnfrom the tube interior and allowed to cool in air to room temperature.Glass deposited in this manner is generally substantially continuous onthe aluminum substrate and has a glossy shiny appearance. The col-orranges from a bright yellow for those compositions containingessentially only arsenic, sulfur and iodine to a dark red for thosecompositions containing substantial amounts of antimony together withthe arsenic, sulfur and iodine.

The thickness of the layer deposited will depend to considerable extenton the time that the aluminum substrate is held in the vapors above themelt. Usually and preferably the thickness of a vapor deposit rangesfrom about 0.07 mil (about 0.00175 mm.) to l mil (about 0.025 mm.) asmeasured with a micrometer caliper though those skilled in the art willappreciate that deposits a little thicker or thinner than this arewithin the spirit and scope of this invention so long as they displaypolarity sensitive switching characteristics when activated (i.e. whenconduction isinduced). After cooling, the layers of glass on aluminumsubstrates are tested for switching and electrical properties in themanner hereinafter described.

Care should be taken not to leave the aluminum substrate in the vaporsof glass too long since then the deposited layer of glass becomesexcessively thick and also the aluminum substrate heats up to meltingtemperatures causing the glass deposit to change in character, assumevariable thicknesses and in general, causing the glass layer produced tobe inoperable for use in making devices of this invention;

An important characteristic of the glass layer is that it can be orderedas seen by X-ray diffraction back reilection analysis with respect tothe aluminum substrate. It is believed that this glass ordering in thethin layers described plays an important role in the switchingcharacteristics in the polarity sensitive characteristics in the devicesdescribed.

As those skilled in the art will appreciate, the term ordered as used inthis application has reference Vto a fixed relationship between theatoms in the glass layer and the atoms in the surface layer of thesubstrate. It is believed, although we do not Wish to be bound bytheory, that the aluminum substrate, whose properties are as describedearlier, aifects the manner in which vapors of the afore-described glasscompositions deposit thereon. Aluminum as described above probably has apreferably oriented surface as respects the metal crystals therein andcauses at least a partial ordering of the vapor deposited glass layers.When these layers are in excess of about 1 mil (about 0.025 mm), we findthat the polarity sensitive switching does not appear to occur and weattribute this to the fact that when the glass layer on the aluminumsubstrate surface exceeds about 1 mil (about 0.025 mm.) in thickness theglass is no longer partially ordered in the manner necessary to producepolarity sensitive switching.

A typical embodiment of this invention is shown in FIGURES l and 2 asdevice 50. A device 50 is mounted for electrical and switching tests asillustrated in FIG- URE l. The switching element of device 50 consistsof ya thin vapor deposited layer 21 of a glass having a composition asabove described upon the substantially oxide free, smooth surface of analuminum substrate 22 (the lower portion of which is broken away inFIGURE 2).

layer 21 is preferably, though not necessarily conveniently p depositeda small amount of a conductive electrode material 25 to provide aposition on the exposed surface of the .glass layer where electricalcontact with the surface can be made of the glass 21 without injurythereto, as with a pointed electrode 24.

A device 50 can be positioned on a support 37 (FIG- URE l) between apointed electrode 24 of tungsten or the like which contacts the glassface 21 through silver deposit 25 and a second pointed electrode 23 ofsimilar construction.

.In FIGURE 4 is shown a schematic circuit diagram illustrating Vtwomeans of inducing conduction in a device 50 of the invention and threemeans for switching a device 50 of the invention. A device 50 is made(ie. in a nonconductive condition) is positioned in the circuit ofFIGURE 4 as showin. An electric field, say of the order of 100 volts permil (about 4000 v/mm.), depending upon the construction 'of device 50,from a voltage source 51 is applied to device 50 by closing the singlepole double throw switch S2 to position 53. A suitable resistor 54having a value of say 50 kiloohrns, depending upon the type of device 50invloved, limits current through the device 50 and acts as a voltagedivider until after it is made semi-conductive.

The device 50 can, alternatively, be made conductive 'by means of acapacitor 55 which is initially charged'vby voltage source 51 by closingthe switch 56. Capacitor 55 is then discharged through resistor 54 andthe device 50 by opening switch 56 and closing single `pole double throwswitch 52 to position 57. Referring to FIGURE 3, the voltage-currentcharacteristic that results from the induced conduction step is shown.Starting at the origin, as positive voltage is applied to device 50 ahigh resistance slope along curve 95 is followed. At a value of sayabout 100 volts per mil (about 4000 v./mm) (point 96) current rapidlyincreases. The curve then follows curve 97 to point 98 during which timethe tra-nsition to a low resistance state rapidly occurs. The voltage isthen decreased to Zero along slope 79.

The device 50, now having induced conduction, is initially in its lowresistance state and can be made to switch to its high resistance stateby application of relatively low voltages as is typical of a device ofthis invention.

The element is switched to its high resistance state from its lowresistance state by closing single pole switch 58 to position 59 so asto cause a small negative voltage from battery 60 to be impressed upondevice 50. The voltage also can be impressed in the form of a pulse.`Continued application of voltage is not required to retain device 50 inits high resistance state.

Device S thus in its high resistance state, is readily switched to itslow resistance state by closing swtich S to position 61 whereby a smallpositive voltage from battery `62 is applied to device 50 with theresult that the device 50 is switched to its low resistance state.Usually the amount of voltage required to switch device 50 from its highresistance state to its low resistance state is greater than thatrequired to switc-h such device 50 from its low resistance state to itshigh resistance state. Continued application of voltage, las in the caseof switching from the low resistance state to the high resistance stateis not required to retain device 50 in its low resistance state.

Switching speed between the respective two resistance states is rapid.For example, when a low voltage pulse of positive polarity as from apulse generator 65 having a pulse duration of one microsecond or less isapplied to the device S0, as by closing the single pole double throwswitch 66 to position 67, device 50 is switched from its high resistancestate to its low resistance state. When device 50 in the low resistancestate is subjected to a 8' negative voltage of short duration, say lmicrosecond or. less from pulse generator 68 by changing single poledouble throw switch 66 from position 67 to position 69 it is switched toa Lhig-h resistance state. Such a pulse switching procedure can Vberepeated indefinitely.l

When device 50 is subjected to low volta-ge alternating current from asignal generator 63 by closing only.` switch 64, device 50 switches toits high resistance state during the negative part of the alternatingcurrent cycle and to its low resistance state during the positive partof the cycle. Thus on 60 cycle alternating current, a device 50 switches120 times, occupying its high resistance state 60 times and its lowresistance state 60 times. At higher frequencies, typically of the orderof 200 killocycles or more, capacitance effects inherently associatedwith a device 50 become pronounced and necessarily interfere with theswitching reliability. This is not a problem, however, when intermittentpulse switching, such as that characteristic of computer circuits, isemployed. Continuous switching with :alternating current serves as aconvenient way to count the number of switches a device 50 makes.

Referring to FIGURE 3 there is seen a plot of the voltage-currentcharacteristic of a device of the invention. If a small negative voltageis applied toa device 50 (assume it to be in its low resistance state),the voltage current plot starts vat the origin and increases linearlyalong curve 70 until point 71 is reached. At this point corresponding toa value of, say about 0.2 volt and 0.8 milliampere, device 50 undergoesan abrupt change of state in a time interval of the order of amicrosecond or even less and passes through a first fast transitionregion 72 t-o a high resistance state designated by point 73. Device 50now follows another linear voltage-current curve 74 and allows currentsonly of the order of a few micro-amps to pass. As the voltage isdecreased to zero and then increased in the positive direction, t'hevoltagecurrent -plct again passes through the origin along curve 75yuntil a value of, say, about 0.3 volt is reached at point 76. At thispoint, device 50 again changes state and rapidly passes through anothertransition region 77. The device 50 is now back in its low resistancestate at point 78 a-nd as the voltage is decreased to zero follows curve79. The voltage-current `characteristic can be retractedV indefinitelyusing the proper specified polarity conventions.

By the term polarity sensitivel switching as used in this application,reference is made to a characteristic transition between high and lowresistance states such that, after a transition from a characteristichigh to a characteristic low resistance state has take-n place in agiven device, or vice versa, in response to an applied electric field ofpredetermined polarity', the polarity of the electric eld required forthe next succeeding transition is opposite to that applied to effectsuch preceding transition. Such switching is further characterized bythe fact that when polarity conventions opposite to those establishedare used for the next succeedi-ng transition (i.e. the polarity of theapplied electric field is the same as that used for the precedingtransition) electric fields about 25 to 50% higher are generallyrequired to effect such a transition. Generally, when such a higherelectric field has been used to effect such a transition, a device ofthe invention no longer switches upon the subsequent application of apulse of the opposite polarity. Characteristically, electric fieldsneeded to effect polarity sensitive switching in accordance with thisinvention are generally at least an order of magnitude and commonly twoorders or more of magnitude smaller than those electric fieldsheretofore `known in the art for effecting so-called symmetricalswitching.` No current limiting or voltage dropping series resistor isgenerally required in polarity sensitive switching.

Polarity sensitive switching characteristics of some devices of thisinvention are given in Table III below:

.individual preference.

TABLE IIL-POLARITY SENSITIVE SWITCHING CHARACTERISTICS OF ORIENTED THINGLASS FILMS Requirements Thickness on Voltage Required Downswtch 1Upswitch 2 Aluminum to Induce Con- High Resistance Low ResistanceExample Substrate duction N o. Voltage Current Voltage Current per-(ma.) perper- (ma.) per- Mils M m. v./mil v./mm. Ohms/ Ohms/ Ohms/ Ohms/Mil Mm. Mil Mm. Mil Mm. Mil Mm.

mil mm. mil mm.

0. 4 0.01 95 3, 800 7)(10 2. 8X108 2)(102 8X103 0. 6 24 0. 01 0. 4 0.520 1. 0 40 0.5 0.0125 60 2, 400 5)(105 2 107 1)(102 4 10s 0.5 20 0. 010. 4 0.5 20 1.1 44 0. 55 0. 0138 70 2, 800 3X105 1. 2)(107 1. 5X102GX10a 1. 2 40 0. 01 0. 4 1. 4 56 2.0 80 0.6 0.015 85 3, 400 3)(101.2X108 5X1()2 2X104 0. 2 8 0. 01 0. 4 0.3 12 0.6 24 0.8 0. 02 65 2, G002)(10x 8X107 9X102 3. 6X104 25 1, 000 0.5 20 2 80 5 200 1 "Downswitchhas refereize to the change which occurs in a glass when it switchesfrom its high resistance state to its low resistance state in responseto an applied electric ii 2 Upswitch has reference to the change whichoccurs in a glass when it switches from its low resistance state to itshigh resistance state in response to an applied electric filed.

8 Composition is not polarity sensitive.

A circuit using a device 50 is shown in FIGURE 5. To one face of device50 a lead wire 8S is attached; the other face, the backside of thedevice, is grounded. This comprises what can be termed the devicesection of the circuit. The program section of the circuit consists of apower source capable of providing positive voltages and negativevoltages to device 50. In the example of FIG- URE the power source isshown to be in the form ot a positive voltage pulse generator 86 and anegative voltage pulse generator 87. It should be understood that thepower source may be any source of alternate polarity electrical energysuch as batteries or the like. The positive voltage source 86 isconnected to a point 88 whic-h is one terminal of a single pole doublethrow switch 89. Likewise, the negative voltage source 87 is connectedto a point 90 which is another terminal of double throw i switch 89. Theother side of t-he positive voltage source 86 and negative voltagesource 87 is grounded to complete the circuit so an input signal can bedelivered to device 50. Double throw switch 89 is conected to device 50through series resistor 91. AOscilloscope 92 is connected across device50 to provide readout of the resistance state of the device.

The operation of this circuit is as follows: Starting Iwith device 50 ina low resistance state such as results from the induced conductionprocedure step previously described, either a positive voltage (denotedwrite-in signal 1 in FIGURE 5) from a pulse `generator 86 or a negativevoltage (denoted write-in signal 0) from pulse generator 87 is appliedto device 50. For the purposes ofV this description, the low resistancestate is assigned the digit l and the high resistance state is assignedthe digit 0,

although t-he opposite coding could be used according to` pulses areunderstood to be suicient to switch the device according to its normalpreferred polarity convention. Assuming that the binary digit l is to bewritten in, the positive voltage pulse is applied to device 50. Since itis already in a low resistance state corresponding to digit l, device150 is not affected by the write-in pulse and remains passive. However,if the binary digit 0 is to be written in, a negative voltage from pulseygenerator 87 is applied to device 50 which is of sufficient magnitudeto switch from a low resistance state to a high resistance state. In thehigh resistance state the binary digit has now been written in. Once theprogram is written into the device 50 it will remain stored until theprogram is changed or in certain instances, when a destructive readoutpulse is applied to said device.

The rea-dout of the device is accomplished as follows: Starting withdevice 50 in a low resistance state as results from the previouslydescribed write-in l step, a positive voltage from pulse generator 86 isapplied to'device 50 by moving switch 89 to terminal 88. The magnitudeof the readout pulses are of the same order as the write-in Themagnitude of the write-ink pulses. Series resistor 91 between switch 89and device 50 is required only for the readout step and is not neededfor the write-in step. Typically, the series resistor should be aboutthe same order of magnitude in resistance as the low resistance state.

Because the removal of series resistor 91 woul-d involve unnecessaryswitching in and out of the circuit it is left in the circuit for bothwrite-in and read out steps. Since the readout pulse is positive and the`device 50 is already in the low resistance state, no switching occurs.The wave form observed on the oscilloscope 92 corresponds to the waveform of pulse generator 86 with no change due to switching of the device50. Now when device 50 is in a high resistance state as results from thepreviously described write-in 0 step, positive voltages from pulsegenerator 86 are again applied to said device. Since the readout pulseis positive and of suiiicient magnitude to switch device 50 to alowresistance state, a characteristic wave form is observed on theoscilloscope. The wave form typically would show the initial voltagedrop of the high resistance (digit l) with an interruption due to rapidswitching and nally the voltage drop of the low resistance state.

The embodiments of the invention in which an exclusive property orprivilege is claimed are deiined as follows:

1. A solid state switch-ing device which, when semiconductive, iscapable of altering its resistance from a high value to a low value andvice versa respons-ive to electric fields of predetermined polarity,said device comprising (a) a substrate of aluminum metal, said substratehaving a surface characterized by (l) having a roughness not greaterthan about 25 microinches (about 0.1625 micron) r.m.s.; and

(2) being substantially free of aluminum oxide;

(b) a coating on at least a portion of said surface, said compositionbeing an ordered vapor deposited layer not more than about l mil (about0.025 mm.) in thickness of a glass composition, the minimum width ofsaid layer being at least twice the thickness thereof; (c) said glasscomposition being one selected from a group of glass systems defined bythe following table:

(d) a pair of electrode means, one contacting said coating, the othercontacting said substrate.

2. A glass composition suitable for use in a device as 'Y defined inclaim 1 comprising:

from about 1 to 10 atomic percent phosphorus,

from about 5 to 35 atomic percent arsenic,

from about 41 to 54 atomic percent sulfur,

from about 2 to 22 atomic percent iodine, and

optionally from 0 to about 35 atomic percent antimony.

3. A glass composition comprising:

from about 1 to l2 atomic percent phosphorus,

from about 20 to 40 atomic percent arsenic,

from about 40 to 60 atomic percent sulfur, and

from about 2 to 20 atomic percent iodine.

4. In a method for switching a polar switching device of the typedescribed `in claim 1 from its characteristic high resistance state toits characteristic low resistance state, the step of applying anelectric iield across a said device suicient to cause same to switchfrom said high resist-ance state to said low resistance state, saidelectric field having an appropriate polarity.

5. In a method for switching a polar switching device References Citedby the Examiner UNITED STATES PATENTS Flaschen et al 106--47 Northoveret al. 106--47 MacAvoy 106--47 Dewald et al 106-47 Stegherr 252-514Mackenzie et al 106-47 2O ANTHONY BARTIS, Primary Examiner.

RICHARD MJWOOD, Examiner.

W. D. BROOKS, Assistant Examiner.

1. A SOLID STATE SWITCHING DEVICE WHICH, WHEN SEMICONDUCTIVE, IS CAPABLEOF ALTERING ITS RESITANCE FROMA HIGH VALUE TO A LOW VALUE AND VICE VERSARESPONSIVE TO ELECTRIC FIELDS OF PREDETERMINED POLARITY, SAID DEVICECOMPRISING (A) A SUBSTRATE OF ALUMINUM METAL, SAID SUBSTRATE HAVING ASURFCE CHARACTERIZED BY (1) HAVING A ROUGHNESS NOT GREATER THAN ABOUT 25MICROINCHES (ABOUT 0.625 MICRON) R.M.S.; AND (2) BEING SUBSTANTIALLYFREE OF ALUMINUM OXIDE; (B) A COATING ON AT LEAST A PORTION OF SAIDSURFACE, SAID COMPOSITON BEING AN ORDERED VAPOR DEPOSITED LAYER NOT MORETHAN ABOUT 1 MIL (ABOUT 0.025 MM.) IN THICKNESS OF A GLASS COMPOSITION,THE MINIMUM WIDTH OF SAID LAYER BEING AT LEAST TWICE THE THICKNESSTHEREOF; (C) SAID GLASS COMPOSITION BEING ONE SELECTED FROM A GROUP OFGLASS SYSTEMS DEFINED BY THE FOLLOWING TABLE: